OHS Code Explanation Guide

Published Date: July 01, 2009
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Part 37 Oil and Gas Wells

Section 779 Drill stem testing

Drill stem testing (DST) is a method of determining the producing potential of a well. The formation fluids exert pressure that is controlled during drilling by using a dense drilling fluid, known as mud, which exerts its own pressure i.e. hydrostatic head, in excess of the formation pressure (see Figure 37.15). DST is the removal of the drilling fluid so that the formation fluids can flow into the now empty drill string.

Figure 37.15 Example of drill stem testing

DST is one of the most hazardous operations within the industry, presenting a unique set of hazards since control is maintained by mechanical and human systems. Guidelines to minimize the probability of failure of either system during a test should be planned and reviewed before any test starts. This plan should include at least

(a) the zones to be tested,
(b) the depths of tests,
(c) the method of testing,
(d) the type of equipment to be used,
(e) the duration of the test, and
(f) a reference to an emergency response plan where applicable.

The emergency response plan should be discussed with all employers and workers involved with the drill stem test. Detailed safe work procedures are described in the Industry Recommended Practice No. 4-2000, Well-Testing and Fluid Handling, published by the Canadian Petroleum Safety Council. In addition, the Petroleum Services Association of Canada (PSAC) has developed the Drill Stem Testing Safety Guideline, which is available on the PSAC website.

Subsection 779(2)

A “mud-can” is a device used to contain fluid and direct it away from the drill pipe when breaking connections (see Figure 37.16). A “test plug” is a valve attached to the top of each length of pipe being pulled from the hole to prevent flow up the drill pipe.

Figure 37.16 Example of a mud can

Subsection 779(3)

No explanation required.

Subsection 779(4)

Since DST has the potential to produce ignitable vapours, any potential sources of ignition must be removed. This can include any pumps, boilers and heaters not required for the operation. Locking out should be considered as an additional safety step.

Subsection 779(5)

Readers are referred to section 188 for information about restraining hoses and piping. Securing can include weights adequately installed to prevent pipe movement. Generally, there should be one weight for each pipe joint.

An alternative method of anchoring is to drill anchors near the pipe ends with a restraining cable running the length of the pipe. The cable system should be continuous and secured at both ends and all individual pressure components should be secured. Restraining cables should not be less than 11 millimetres in diameter or chains of equal or greater strength should be used.

Subsection 779(6)

As long as there is adequate lighting, DST may be conducted during darkness until hydrocarbons appear at the surface. At this point the recovery must be reverse-circulated. Reverse circulation is the intentional pumping of fluids down the annulus i.e. area of the well bore that is outside the drill pipe, and back up through the drill pipe. This is the opposite of the normal direction of fluid circulation in a well bore. A pump-out sub is typically in the test string in order to reverse.

Reverse circulation requires proper disposal of the contents of the drill string by pumping to a tank or a vacuum truck. The receiving vessel must be properly grounded and vented with any engines turned off. Extra care must be taken once the pump-out sub has reached the rig floor since hydrocarbons may be present below the pump-out sub.